Gas turbine engines are subjected to high temperatures during operation and consequently include components made from refractory materials. An oxide-based ceramic matrix composite (CMC) substrate (hereinafter “oxide-based CMC substrate(s)”) is one example of a high temperature structural material used for a gas turbine engine component. Such oxide-based CMC substrates have a tendency to wear at a wear surface when placed in motional or vibratory contact with metals. Such wear may cause undesirable recession, dimensional changes, loss of mass, and corresponding undesirable degradation of mechanical and thermal properties of such composites and the components and systems in which they are used.
As an example, structural components made from CMC substrates, including oxide-based CMC substrates, may be exposed to such contact when held in a metal fixture or bracket within a larger subsystem, e.g., in an engine, and more specifically in an engine which is part of an airborne system, such as an airplane. In the latter environment, the CMC substrates and metal components may also be subject to a wide range of temperatures, from below freezing 0° C. (32° F.) to above 1000° C. (1832° F.). The chemical environment may be oxidizing, resulting in the formation of oxides on the metal surfaces.
Further, for certain oxide-based CMC substrate fabric architectures, for example, a two-dimensional (2-D) fabric layup, the wear rate of the x-y, or fabric plane is highly variable with increasing contact time and/or distance traveled during the wear process. This variability in the wear rate of the oxide-based CMC substrate is due to the discrete layered structure in the thickness (or z) direction. The wear rate of unprotected oxide-based CMC substrate surfaces is therefore large, extremely variable, and not readily predictable at any given starting or intermediate condition of the wear process, when the two wear surfaces are first brought in contact or at a later stage.
The surfaces of oxide-based CMC substrates may also require protection against other materials or conditions which may be present in the use environment as described, or in other environments, for example against undesirable erosion and recession from particles. Current, commercially available oxide-based CMC substrates, fabricated by a single cycle of liquid slurry infiltration and annealing, are highly porous, with open and accessible porosity at external surfaces. This porosity aggravates the sensitivity to such harsh environments.
Ceramic matrix composite (CMC) substrates, including oxide-based ceramic matrix composite (CMC) substrates are difficult to coat successfully. Typical oxide-based CMC substrates are relatively inert chemically, and usually have no significant surface features to provide any type of mechanical lock with the coating. Furthermore, typical commercially available oxide-based CMC substrates have high (30-50%) matrix porosity, and the open, small accessible pores at the surfaces of these CMC substrates make coating from a liquid precursor very difficult. It is especially difficult to get solid coatings to adhere to oxide-based CMC substrates during direct and sustained mechanical contact, and remain adherent over the required temperature range in harsh environmental conditions. In general, the level of adhesion required from a coating subjected to mechanical stresses due to direct and sustained mechanical contact, such as in wear applications, is much higher than the level of adhesion required in applications where no direct mechanical loads are applied to the coating, such as in environmental or oxidative barrier (EBC/OBC) applications.
While bond coats are used with some coatings to ensure adhesion of the coating to a substrate surface, their use just introduces another possible failure point and complicates the coating process. A single, adherent coating which does not require a bond coat is extremely advantageous from practicality and cost effectiveness points of view. Similarly, substrate surface treatment is often performed prior to coating in order to improve adhesion. However, such surface treatment may undesirably reduce the mechanical and other functional properties of the oxide-based CMC substrates.
Accordingly, it is desirable to provide protective coatings that substantially protect CMC substrates from wear, recession, and erosion, and that are suitable for high temperature and oxidative environments, and adhere to the surface of the CMC substrate during direct and sustained mechanical contact in such environments and remain adherent over the required temperature range, and without a bond coat. In addition, it is desirable to reduce the variability in wear behavior of uncoated oxide-based CMC substrates, so as to obtain a material surface with a low and relatively constant and predictable wear rate. It is also desirable that the coating be stable, and not evaporate or decompose in the contemplated use environments. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.